During crystallisation, the particle size distribution of the crystals depends mainly on the following processes:
nucleation, PA0 crystal growth, PA0 attrition, PA0 agglomeration, PA0 maturation of the crystals. PA0 a) production of a concentrated syrup, PA0 b) reduction of pressure PA0 c) evaporation under reduced pressure with vigorous agitation in the crystallisation zone until crystals appear, PA0 d) stopping evaporation and maintaining agitation for a certain period, PA0 e) resumption of evaporation and agitation until a dry product is obtained, PA0 the particle size follows a Gaussian distribution the median of which is about 20 to about 220 .mu.m and particularly about 20 .mu.m to about 200 .mu.m, the coefficient of variation being about 20% to about 50%, particularly about 30% to 45%, or about 35% to 45%, or about 30% to 40%, or PA0 the particle size distribution is characterised by a uniformity index between 1 and 5, particularly between 2.5 and 3.5. PA0 its rate of dissolution is about 5 to about 10, particularly about 7 to about 9 seconds, under the following conditions: 10 g of composition per 100 ml of pure demineralised water, at a temperature of 18.degree. C., PA0 it is non-caking, PA0 its pourability index is greater than about 80, and ranges from about 80 to about 85, particularly from about 81 to about 82, measured according to the Hosakawa test, as described in IRON WORKS LTD., Osaka, Japan, and if it is glucose, the pourability index is about 55 to about 70, PA0 the specific gravity of the compacted product is about 0.90 to about 1.00, particularly about 0.97 to about 1.00, and the specific gravity of the uncompacted product is about 0.75 to about 0.90, particularly about 0.83 to about 0.87, measured according to the Hosakawa test, and if the said product is glucose, the specific gravity of the compacted product is about 0.70 to about 0.90 and the specific gravity of the uncompacted product is about 0.50 to about 0.70. PA0 a) a concentrated sucrose syrup with about 60 to about 97, particularly 75% by weight of dry matter is prepared, PA0 b) the pressure is reduced from atmospheric pressure to a value of about 100 to about 300 mbars, particularly about 200 mbars, in order to start evaporating a part of the water contained in the sugar syrup, the rate of evaporation being about 20%, PA0 c) a part of the water contained in the sugar syrup is evaporated under reduced pressure (about 200 mbars) and the syrup is stirred, particularly by mechanical agitation at a peripheral speed of about 100 to about 350 m/min, particularly 200 to 350 m/min, until a coefficient of sugar supersaturation between 1 and 1.3, particularly 1.1 and 1.3 is obtained, and crystallisation is brought about by vigorous agitation of the syrup (in addition to the stirring mentioned above), particularly by mechanical shocks generated by beating by impact, in this supersaturation zone. PA0 d) crystallisation is continued by stopping evaporation and vigorous agitation (beating), and maintaining regular agitation (stirring) for the time required to obtain crystals of the desired size, and advantageously for about 5 min to about 20 min, PA0 e) evaporation is resumed (still whilst stirring the medium at a speed of about 100 to about 250 m/min) until crystals containing less than 1%, particularly less than 0.5% moisture are obtained, PA0 the temperature being kept at a value of about 70.degree. C. to about 100.degree. C. throughout the process, and the pressure being advantageously kept at about 200 mbars during steps c) to e). PA0 a) a concentrated sucrose syrup is obtained with about 60 to about 97, particularly 75% by weight of dry matter, PA0 b) the pressure is reduced from atmospheric pressure to a value of about 100 to about 300 mbars, particularly about 200 mbars, in order to start evaporating a part of the water contained in the sugar syrup, the rate of evaporation being about 20%, PA0 c) a part of the water contained in the sugar syrup is evaporated under reduced pressure (about 200 mbars) and the syrup is stirred, particularly by mechanical agitation at a peripheral speed of about 100 to about 350 m/min, particularly 200 to 350 m/min, until a coefficient of sugar supersaturation between 1 and 1.3, particularly 1.1 and 1.3 is obtained, and crystallisation is brought about by vigorous agitation of the syrup (in addition to the stirring mentioned above), particularly by mechanical shocks generated by beating by impact, in this supersaturation zone, PA0 d) crystallisation is continued by stopping evaporation and vigorous agitation (beating), and maintaining regular agitation (stirring) for the time required to obtain crystals of the desired size, and advantageously for about 5 min to about 20 min, PA0 e) evaporation is resumed (still whilst stirring the medium at a speed of about 100 to about 350 m/min, particularly 200 to 350 m/min), until crystals containing less than 1%, particularly less than 0.5% moisture are obtained, PA0 the temperature being kept at a value of about 40.degree. C. to about 100.degree. C., particularly about 70.degree. C. to about 100.degree. C. throughout the process, and the pressure being advantageously kept at about 200 mbars during steps c) to e). PA0 a) a concentrated syrup is prepared, PA0 b) the syrup is evaporated under pressure with vigorous agitation in the crystallisation zone until crystals appear, the temperature and the rate of evaporation being controlled until a dry matter content of about 80% to about 90% is obtained, PA0 c) evaporation is continued with a reduction in the rate of agitation until a dry product is obtained, the temperature being kept constant with respect to the previous step, PA0 the temperature being adjusted and kept at a given value in the interval from about 40.degree. C. to about 100.degree. C., and particularly from about 70.degree. C. to about 100.degree. C., during steps a) to c) described above. PA0 a) a concentrated sugar syrup is prepared with about 60% to about 97%, particularly about 75% by weight of dry matter, PA0 b) evaporation of the syrup is brought about by reducing the pressure so as to achieve boiling of this syrup at the chosen temperature, PA0 c) the syrup is stirred particularly by mechanical agitation at a peripheral speed of about 100 to about 350 m/min, particularly 200 to 350 m/min, the coefficient of supersaturation of the syrup being between 1 and 1.3, particularly 1.1 and 1.3, and crystallisation is brought about by vigorous agitation of the syrup (in addition to the stirring mentioned above), particularly by mechanical shocks generated by beating by impact, in this supersaturation zone, PA0 d) evaporation is continued under the same temperature and pressure conditions as those used in the previous steps, until a medium is obtained of which the crystals constitute the majority phase (more than about 50%, and particularly more than about 70% with respect to the medium), PA0 the rate of agitation being reduced by about 50 to about 200 m/min, the temperature being kept constant with respect to the previous steps, beating being maintained until a dry product is obtained, composed of crystals of the desired size, containing less than 1%, particularly less than 0.5% of moisture, PA0 the temperature being adjusted and kept at a constant value within the range from about 40.degree. C. to about 100.degree. C., particularly about 70.degree. C. to about 100.degree. C., throughout the duration of the steps. PA0 a) a concentrated syrup is prepared, PA0 b) the syrup is evaporated under pressure with vigorous agitation in the crystallisation zone until crystals appear, the temperature and the rate of evaporation being controlled until a dry matter content of about 80% to about 90% is obtained, PA0 c) evaporation is continued with a reduction in the rate of agitation until a dry product is obtained, the temperature being kept constant with respect to the previous step, PA0 the temperature being adjusted and kept at a given value in the interval from about 40.degree. C. to about 100.degree. C., and particularly from about 70.degree. C. to about 100.degree. C., during steps a) to c) described above. PA0 a) a concentrated sugar syrup is prepared with about 60% to about 97%, particularly 75% by weight of dry matter, PA0 b) evaporation of the syrup is brought about by reducing the pressure so as to achieve boiling of this syrup at the chosen temperature, PA0 c) the syrup is stirred particularly by mechanical agitation at a peripheral speed of about 100 to about 350 m/min, particularly 200 to 350 m/min, the coefficient of supersaturation of the syrup being between 1 and 1.3, particularly 1.1 and 1.3, and crystallisation is brought about by vigorous agitation of the syrup (in addition to the stirring mentioned above), particularly by mechanical shocks generated by beating by impact, in this supersaturation zone, PA0 d) evaporation is continued under the same temperature and pressure conditions as those used in the previous steps, until a medium is obtained of which the crystals constitute the majority phase (more than about 50%, and particularly more than about 70% with respect to the medium), PA0 the rate of agitation being reduced by about 50 to about 200 m/min, the temperature being kept constant with respect to the previous steps, beating being maintained until a dry product is obtained, composed of crystals of the desired size, containing less than 1%, particularly less than 0.5% of moisture, PA0 the temperature being adjusted and kept at a constant value within the range from about 40.degree. C. to about 100.degree. C., particularly about 70.degree. C. to about 100.degree. C., throughout the duration of the steps. PA0 suppression of step d): "stopping evaporation and maintaining agitation for a certain period", PA0 process II is advantageously applied to industrial trials.
In order to obtain a large quantity of regular crystals with a fine particle size, it is necessary to apply a process which promotes nucleation more than crystal growth. To do this it is necessary to use appropriate means permitting good control of the crystallisation parameters.
A large quantity of sugar crystals with a regular shape and a very fine particle size cannot be obtained directly by the existing crystallisation processes. In the production of various types of sugar, a process has been developed which is better known as a conversion process. This process is used for the production of sugar in the form of a non-caking, free-flowing, granulated powder which is readily dispersed in aqueous solution. This process has been described at length in several patents.
U.S. Pat. No. 3,194,682 (Tippens et al.) describes a process using a syrup concentrated to 95-97 brix (% by weight of dry matter) at 121-129.degree. C. which undergoes rapid cooling with vigorous agitation. This method allows the production of agglomerates of which the sugar crystals are of melting size (3-50 microns).
U.S. Pat. No. 3,365,331 (Miller et al.) describes a similar process which leads to the production of agglomerates. In this case, the crystals are obtained by beating a super-saturated syrup.
In patent EP 0 052 413, the beating process at a well controlled temperature allows heat-sensitive compounds to be incorporated in the final product.
All the processes described lead to a powder of fine-grained sugar. The granules have an irregular shape giving low density powders. As the particle size is selected by sieving, the yield of one class of powder is thus low. There is, therefore, a need to develop a process that allows the production, with good yields, of regular crystals with a fine particle size, this need being met by the present invention.